1. Field of the Invention
The invention concerns a process and an apparatus for vault-structuring, in which curved material sheets are pressurized using supporting elements spaced at certain distances from each other.
2. Background Art
Numerous processes for profiling thin material sheets are known, including well-known deformation technologies such as rolling-in or embossing of beads with the aid of complicated form tools to create three-dimensional stiffening. The drawback of these mechanical deformation processes is that sophisticated and expensive form tools are required, that the material sheets to be profiled are heavily plastified, and that the surface quality of the raw material is degraded by the mechanical surface pressure.
The European patent application 0 441 618 A 1 describes a profiling technique in which polyhedral structures are produced with the aid of two embossing rolls. An apparatus is known for embossing axial beads into cans by supporting the can by axial, rigid elements on the inside and applying pressure from the outside by means of an elastic press roller (DE 35 87 768 T 2). U.S. Pat. No. 4,576,669 suggests feeding plastic foil over a roll that carries small cups into which the plastic foil is sucked by vacuum pressure. This process, however, does not enhance the inherent stability of the material. A process in which round, dome shaped structures are impressed in a foil does likewise not considerably improve the inherent stability of the material because large regions remain undeformed between the dents (French patent application no. 1,283,530).
Furthermore, a process is known in which thin material sheets or foils are profiled dent-like. In the process, the curved thin material sheet or foil is supported by line-shaped supporting elements on the inside and hydraulically pressurized from the outside. Offset quadrangular dent structures result that considerably improve the inherent stability of the material sheet (Deutsche Offenlegungsschrift 25 57 215 [=Patent Application Open To Public Inspection], German printed patent specification DE 43 11 978). In principle, this dent-profiling process differs from the one described in patent application no. 0 441 618 A 1 in that not two mechanically acting embossing rollers are required but only a supporting core on which the material sheet rests and against which it is hydraulically pressed. The hydraulic production of polyhedral structures, e.g. hexagonal profiles, has been described in the International Patent Application published as PCT/EP 94/01043, FIGS. 5b and 5c). Instead of hydraulic pressure, an unprofiled, elastic cushion or an unprofiled elastomer can be used for pressurization. The supporting elements against which the material sheet is pressed are made of a flexible material which is either fixed or can move on the core.
The purely mechanical forming process described in the European printed patent specification no. 0 441 618 A 1 considerably affects the surface quality of the raw material by great mechanical deformation. The apparatus for producing axial beads described in DE 35 87 768 T 2 uses line-shaped, axial, rigid supporting elements and an elastic pressure element. However, the inherent stability of the material furnished with axial beads in this way is insufficient because, for geometrical reasons, beads do not yield multi-dimensional inherent stability. In contrast to beads the forming techniques described in O.S. 25 57 215, DE 43 11 978, and PCT/EP 94/01043 produce multi-dimensional inherent stability by creating offset vault structures without degrading the surface quality.
One of the problems of the known vault-structuring techniques is that, where the vaults in the profiled material are deep, local stretching and elongation occur, which can be so severe that considerable plastic deformation results, weakening the material so that it may tear.
Another problem of the known vault-structuring techniques for thin material sheets or foils is that the self-organizing of the folds that bring about the improvement of the inherent stability is not, or only insufficiently, made possible in some applications. Self-organization of the vault folds means a process in which the material is folded in several dimensions in a such way that its inherent stability is enhanced. For example, such vault-structuring is effected by the curved, thin material, which is supported on the inside by supporting collars spaced at certain distances from each other or a helical, rigid supporting spiral (O.S. 25 57 215), becoming instable due to external pressure. The instability triggers multi-dimensional folding of the material and offset, quadrangular vault structures are created. Thus the thin material is transferred into a new state, the most important characteristic feature of which is its improved inherent stability. One problem of these quadrangular vault structures is that severe plastic deformation may occur in the region of the vault folds which weaken the material. If, instead of rigid ones, flexible supporting elements (e.g. of rubber) are used which are allowed to move on a core in axial direction during the vault-structuring process, hexagonal vault structures are created. Such hexagonal vaults can also be produced by hexagonal, rigid supporting elements (PCT/EP 94/01043). Studies have shown that severe plastic deformation weakening the material may occur in the area of the hexagonal vault structures as well, similar as in the case of the offset, quadrangular vault structures. In addition, the material sheets thus profiled with quadrangular or hexagonal vault structures is difficult to flatten from the cylindrical into a flat shape without substantial loss of isotropic inherent stability. Studies have shown that the lateral vault folds arranged in the direction of feed of the material sheet can be bent into a flat shape only by application of considerable force. Because in this flattening process the vault folds perpendicular to the direction of feed of the material sheet are leveled and arched somewhat, the vault folds lose a portion of their initial inherent stability. The thicker the material sheets the more serious the problem, and no isotropic inherent stability of the profiled material sheets can be achieved in this way. Therefore, the known profiling techniques are limited to angular structures such as quadrangular and hexagonal profiles. Due to this limitation, the structure of the vault folds could not yet be optimized. Such optimization includes the geometry of the structure as well as the geometrical shape of the fold itself. The structure of the vaults, such as their size and depth, determines the increase in inherent stability at a given thickness of the material. The contours of the folds must adopt such a shape that despite their being smoothed only a minimum of plastic deformation occurs.